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Q2971. Can value be null in TreeMap ?

Core Java

Ans. Yes.

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Q2972. Which interface TreeMap implements ?

Ans. TreeMap implements NavigableMap, SortedMap, Serializable and Clonable.

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Q2973. Do we have form beans in Struts 2 ?

Ans. No, because they are not longer required. As action classes are no more singleton in Struts 2, user inputs can be captured in action itself.

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Q2974. What is a ConcurrentHashMap ?

Core Java

Ans. ConcurrentHashMap is a hashMap that allows concurrent modifications from multiple threads as there can be multiple locks on the same hashmap.

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Q2975. What is the difference between these two approaches of creating singleton Class ?

//Double Checked Locking Code
public static Singleton createInstance() {
   if(singleton == null){
      synchronized(Singleton.class) {
         if(singleton == null) {
            singleton = new Singleton();
         }
      }
   }
   return singleton;
}

//Single checked locking code
public static Singleton createInstance() {
   synchronized(Singleton.class) {
      if(singleton == null) {
         singleton = new Singleton();
      }
   }
   return singleton;
}

Design

Ans. In First Case , Lock for the synchronized block will be received only if singleton == null whereas in second case every thread will acquire the lock before executing the code.
The problem of synchronization with singleton will only happen when the object has not be instantiated. Once instantiated , the check singleton == null will always generate true and the same object will be returned and hence no problem. First condition will make sure that synchronized access ( acquiring locks ) will only take place if the object has not been created so far.

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Q2976. Why are Getter and Setter better than directly changing and retrieving fields ?

or

What is the significance of variable hiding in Java ?

or

Why it's preferred to keep variables private and allow their access through getter and setter methods ?

Core Java

Ans. 1. Methods can participate in runtime polymorphism whereas member variables cannot.

For example -

Vehicle vehicle = new Car();

car.getObjVariable();// will return variable defined in Car

whereas

car.variable

will return the variable from Car class irrespective of the object it holds.

2. Validations can be performed before setting the variables.

3. If the input format changes or some other value or calculated value needs to be returned , that can be absorbed by making change ( wrapping ) in the setter and getter. By this the call to method from outside or interface to the object will remain the same.

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Q2977. Can we overload main method in Java ?

Core Java

Ans. Yes, but the overloaded main methods without single String[] argument doesn't get any special status by the JVM. They are just another methods that needs to be called explicitly.

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Frequently asked in Phone Interviews.

Q2978. What the Bean scopes provided by Spring ?

Spring

Ans. Singleton , Prototype , Request , Session , global-session

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Frequently asked in Phone Interviews.

Q2979. What are the various Auto Wiring types in Spring ?

Ans. By Name , By Type and Constructor.

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Frequently asked question in companies using hibernate.

Q2980. Difference between first level and second level cache in hibernate ?

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